1. Field of the Invention
The present invention relates to an apparatus for searching for a cell and a method of acquiring a code unique to each cell in an asynchronous wideband Direct-Sequence Code Division Multiple Access (DS/CDMA) receiver.
2. Description of the Related Art
An asynchronous wideband DS/CDMA system is one of the International Mobile Telecommunication (IMT)-2000 standard recommendations and it is expected to play an important role in the next generation of mobile communications. There are two types of systems in a DS/CDMA system: a synchronous system and an asynchronous system. The synchronous system synchronizes cells using an external timing source such as a global positioning system. On the other hand, the asynchronous system does not synchronize cells.
In a DS/CDMA system, cells are distinguished from each other by a spreading code. In a synchronous system capable of controlling the synchronization of cells, a spreading code with a different phase is assigned to each cell. In other words, a single unique spreading code is used. On the other hand, an asynchronous system, which does not use time information, assigns a different spreading code to each cell.
In a DS/CDMA system, a cell search indicates a procedure in which a mobile station (MS) searches for an optimal cell when the communication between the MS and a base station (BS) begins and acquires a spreading code assigned to the cell and the synchronization of the code. It is usually more difficult and takes more time to search for a cell in an asynchronous system than in a synchronous system. In a synchronous system in which the same spreading code is assigned to all cells, the only process that must be done is to obtain the phase of the code. In the asynchronous system, in addition to the phase of the spreading code, the code sequence should be obtained. Therefore, the cell search is very difficult and important in an asynchronous wideband CDMA system.
In a wideband CDMA system, spreading codes distinguishing cells are referred to as scrambling codes. There are 512 scrambling codes. If all of the 512 scrambling codes must be searched during a cell search, it requires extensive time and processing. To overcome this problem, the conceptions of a code group and a Synchronous CHannel (SCH) are applied to the wideband CDMA system. A code group is one of the groups into which the scrambling codes are divided. Each cell is allocated a unique code group. As a result, the number of scrambling codes to be searched by a MS can be decreased. Sixty-four (64) code groups exist in the wideband CDMA system, and eight (8) scrambling codes are allocated to each code group. Accordingly, once a code group is detected during a cell search, the number of scrambling codes to be searched is decreased to 8. A code group allocated to each cell is determined by a SCH.
A SCH, which is a downlink physical channel used for a cell search, is time-multiplexed with a primary Common Control Physical CHannel (p-CCPCH) and transmitted at every slot. The SCH is a kind of a control channel and is composed of a Primary Synchronization Code (PSC) and a secondary synchronization code (SSC). The PSC and the SSC are simultaneously transmitted at every slot.
FIGS. 1(a) through 1(c) illustrate a SCH in a hierarchical structure. FIG. 1(a) illustrates a single superframe composed of 72 frames. The duration of a single super frame is 720 ms. FIG. 1(b) illustrates a single frame composed of 15 slots. The duration of a single frame is 10 ms. FIG. 1(c) illustrates a single slot composed of a p-CCPCH composed of 9 symbols and a PSC Cp and a SSC Csi which are each composed of one symbol. The duration of a single slot is 0.667 ms, and one symbol is composed of 256 chips.
Each of the PSC and the SSC is a code sequence composed of 256 chips. In a wideband CDMA system, there is a single unique PSC common for every cell and there are 16 different SSCs for 15 slots in one frame. In addition, the PSC is orthogonal to the SSC. A cell search is performed using a SCH. The boundary of a slot is found through the slot synchronization, and a code group is identified based on the correlation between the SSC and a received signal. Once the code group is identified, a scrambling code allocated to each cell is detected.
Accordingly, it is necessary to exactly detect a SSC transmitted through the SCH of each slot for identifying a code group. In an ideal environment, for each of the 15 received slots, the SCH signal of the received slot is correlated with the 16 SSCs, and the SSC that gives the maximum correlation value is detected. However, under actual conditions, the performance is degraded due to various factors. The primary factors contributing to the degradation of performance are noise, a change in a channel due to the movement of a MS, a frequency error due to a mismatch between the oscillators of a transmitter and a receiver and the like.
Among the above factors, the frequency error is an unavoidable phenomenon due to the physical characteristics of a device in a communication system and considerably degrades the performance of the system. Particularly in a wideband DS/CDMA system in which a cell search is performed by correlating a received signal with various codes, a frequency error exerts a negative influence on a correlation property. As a result, a cell search probability decreases, and a cell search time increases.
To minimize the degradation of performance due to the above factors, the results of correlations between a received signal and SSCs are combined with each other throughout a plurality of frames. A combining method includes a coherent combining method and a noncoherent combining method. The coherent combining method is robust to noise but degrades the performance more severely when a channel state is changed severely or a frequency error occurs. On the other hand, the noncoherent combining method is robust to a channel state change and a frequency error but severely degrades the performance when a signal-to-noise ratio (SNR) is decreased due to significant noise. In an ideal additive white Gaussian noise (AWGN) channel without a frequency error, the performance of the coherent combining method is superior to that of the noncoherent combining method by about 3 dB. Accordingly, it is preferable to remove a frequency error and the influence of a channel from a received signal and use the coherent combining method.